provision: Make sure the correct version of pip is installed.

pip of at least 8.0.0 version is required to successfully install the
requirements. Otherwise, the prefix option doesn't work.

Author: alenavolk

tools/provision

@@ -57,10 +57,12 @@ the Python version this command is executed with."""
 
     # In order to install all required packages for the venv, `pip` needs to be executed by
     # the venv's Python interpreter. `--prefix venv_dir` ensures that all modules are installed
-    # in the right place.
+    # in the right place (pip of at least 8.0.0 version is required for this to work).
     def install_dependencies(requirements_filename):
-        if subprocess.call([os.path.join(venv_dir, venv_exec_dir, 'pip'),
-                           'install', '--prefix', venv_dir, '-r', os.path.join(base_dir, requirements_filename)]):
+        pip_path = os.path.join(venv_dir, venv_exec_dir, 'pip')
+        subprocess.call([pip_path, 'install', 'pip>=8.0.0'])
+        if subprocess.call([pip_path, 'install', '--prefix', venv_dir, '-r',
+                            os.path.join(base_dir, requirements_filename)]):
             raise OSError("The command `pip install -r {}` failed. Dependencies not installed!"
                           .format(os.path.join(base_dir, requirements_filename)))